For many years, compressed asbestos sheet material has been used for gasketing and a wide range of other industrial uses. Compressed asbestos sheets have traditionally been prepared by mixing a base of asbestos fibers with a rubber binder and subjecting the resultant mixture to pressure and elevated temperature. Typically, known asbestos sheets can be made on a two roll sheeter machine (such as a Troester machine) which has a pair of rollers located vertically one above the other. The lower roller is a larger, hot roller while the upper roller is a smaller, normally unheated roller. To form the conventional asbestos sheet on a machine of this type, a quantity of starter compound is first built up on the hot roller and then a quantity of body compound is added in the nip between the two rotating rollers. A compressed sheet material will be formed on the hot roller to the desired thickness and may then be removed and placed on top of a table or other support.
In recent years, the issue of possible health problems caused by the adverse influence of asbestos on the human body has led to the development of non-asbestos gasketing sheet materials. For example, U.S. Pat. No. 4,271,228 to Foster et al. discloses a flexible sheet material particularly suitable for use in gaskets which includes organic fiber, organic binder, and a blend of finely divided vermiculite with another asbestos-free inorganic finely divided particulate filler. This use of organic fibers in place of asbestos for the formation of gasket material is also illustrated by the gasket material compositions disclosed in the Lancaster et al. U.S. Pat. Nos. 4,529,662 and 4,529,663. Here, a combination of organic cellulose fibers are mixed with organic amide fibers to form the fiber base for the gasket material.
Some non-asbestos gasketing materials have been developed which incorporate a combination of organic and inorganic fibers as the base material for the gasket, and U.S. Pat. Nos. 4,508,777 to Yamamoto et al. and 4,656,085 to Bechen et al. are illustrative of this art. It should be noted that Yamamoto et al. discloses both the combination of two types of organic fibers or a combination of an organic with an inorganic fiber, and in some cases the inorganic fiber employed is a carbon fiber. This use of carbon fibers is also disclosed in the gasket sheet of the Tsuchimoto et al. U.S. Pat. No. 4,546,033. Here, the gasket sheet is formed from 50% to 85% by weight of a fibrous material, which may be carbon fiber or a mixture of organic and inorganic fibers, and by a rubber material binder.
Current non-asbestos gasketing products manufactured on a two roll sheeter or calendar have been found to operate satisfactorily at temperatures up to approximately 700 degrees F. However, at temperature ranges from 700 degrees F to 1000 degrees F., gasketing materials formed from organic fibers have proven to be unsatisfactory. Carbon fibers will satisfactorily withstand high temperatures when employed as a base for a gasketing material, but it has been generally found that, for the most part, carbon fibers are either too brittle or of too low a carbon content to provide effective heat resistance. For example, the carbon fibers disclosed in the aforementioned Yamamoto et al patent have a carbonization level of about 80%. With most carbon fibers, the higher the percentage of carbonization, the higher the modulus of elasticity, and this modulus, for many carbon fibers, is in the range of 25,000,000 to 30,000,000 psi. The use of fibers of this type results in gasketing products which are too brittle, have poor tear resistance, and in general exhibit poor physical properties.
The non-asbestos compressed sheet materials which have been previously developed for gasket applications have generally included large percentages by weight of organic, inorganic or mixed fibers, and, as indicated by the aforementioned Yamamoto et al patent, with such previous formulations, fiber amounts of less than 25% by weight result in sheets with decreased tensile strength. Also, such previous formulations often employ large percentages of organic fibers, such as aramid, which inhibit the ability of the sheet to operate effectively as a gasket material in high temperature environments, particularly for chemical applications. Consequently, a need exists for a high temperature, non-asbestos sheet material having a fiber base formed primarily of carbon fibers which impart effective temperature rresistance to the sheet while providing a sheet with excellent sealing effectiveness and tensile strength. Ideally, a small portion of organic fiber is incorporated in the fiber base to enhance the tensile strength of the sheet, but the proportion of organic to carbon fiber must be such that the heat resistance and sealing capabilities of the material are not degraded, even in chemical environments.